This is probably to be explained by the growth of the medullary fold at this point, which pushes back the primitive groove.
A large part of the wood is formed by the medullary or pith rays.
The second (fig. 14) section passes through the medullary groove where it has become wider.
medullary rays are numerous, but very thin and inconspicuous.
medullary shell ellipsoidal (with longitudinal main axis), one-third as broad as the cortical shell.
The medullary rays of redwood are thin and very obscure, but numerous.
In sections its front end appears as a ridge on one side or on the middle of the floor of the widened end of the medullary groove.
The medullary rays are broad, but not numerous in comparison with white oak.
Cortical shell thin walled, with rough surface, three times as broad as the medullary shell.
The medullary rays are broad, in the heart black, in the sapwood white.